This patent specification is in the field of remote monitoring of biomedical data, such as ECG (electrocardiographam) data.
Transtelephonic monitoring of cardiographic data such as ECG waveforms and heart pacer information has been used for many years. Typically, a cardiac transducer at a patient""s home produces an electrical ECG signal in the form of a voltage across a pair of ECG pads that are in electrical contact with the patient""s body. The resulting ECG waveform is used to frequency modulate a carrier, and the resulting FM signal drives a speaker producing an acoustic FM signal played into the mouthpiece of a telephone receiver that converts the acoustic signal back to an electrical FM signal. Via the telephone network, a central station receives the transmitted signal and processes it to reconstruct, display, and record the ECG waveform or to extract other information. If the cardiac signal is pacer related, the information of interest could be the duration of a pacer pulse or the time between pulses. The conversion to an acoustic signal and back to an electrical signal can be avoided if the patient has suitable equipment and skill for the purpose. Examples of transtelephonic monitoring of cardiac information can be found in U.S. Pat. Nos. 4,938,229 and 5,467,773 (each incorporated by reference herein), and 5,735,285, as well as in references cited in said patents.
The assignee of this patent specification supplies such equipment and services, described further at its Website. Typically, information from different ECG pads or combinations of pads (vectors), or from a pacer, is embedded in the FM signal serially, and packets of additional information such as device ID and time stamps usually are inserted. Only one ECG waveform, or only one pacer pulse, or only one item of identifying information, modulates the carrier at any one time. It is believed that several years ago an entity called the Cardiac Evaluation Center in Milwaukee, Wis. offered, and may still be offering, a two-channel transmitter and a proprietary receiver, encoding two ECG waveforms at the same time into a single FM signal that is separated at a proprietary receiver believed to have used analog bandpass filters for the separation. It is not known what technique that system used to demodulate the FM signal.
In the two patents incorporated by reference herein, the FM signal was demodulated at the receiving station to extract the ECG waveform by finding the zero crossings of the FM signal and measuring the time between those zero crossings. In particular, the patented systems counted a clock during the intervals between adjacent zero crossings and converted the counts to frequency, thereby reconstructing the original ECG waveform. U.S. Pat. No. 5,735,285 is understood to propose another zero crossing detection technique, involving an examination of the area where digitized samples of the FM signal transition between positive and negative values. While such zero crossing analysis of the FM signal, with appropriate suppression of noise and other sources of inaccuracies, has been used for years, it is believed that a need still remains for: (1) a more accurate and reliable reconstruction of the original ECG waveform; (2) the simultaneous transmission of multiple ECG waveforms or other information coupled with such more accurate and reliable reconstruction of the original information; and (3) such simultaneous transmission demodulated at the receiving station using a general purpose computer that can be conveniently adapted through programming to different formats of information transmission and can be less costly and more acceptable than proprietary hardware.
This patent specification discloses a system and a method for remotely monitoring, at a central station, cardiac conditions existing at a remote station. In a preferred embodiment, three or more ECG waveforms are derived from a patient at a local station. These ECG waveforms frequency modulate respective different carriers to thereby produce three or more respective FM signals. These FM signals are combined into a composite FM signal containing concurrent information from the three or more ECG waveforms, and are transmitted to a central station. At the central station, the received composite FM signal is processed both in the time domain and in the frequency domain to reconstruct the three or more individual ECG waveforms in a manner that comprises estimating local frequencies at portions of the composite FM signal that are substantially closer to each other than zero crossovers of the composite FM signal. The process calculates local phase differences and uses them to estimate said local frequencies. The local phase differences are calculated by combining digital samples of the composite FM signal with a phase shifted version of the digital samples.